The lawyer who acts for the author Harper Lee has spoken about the controversy surrounding a recently discovered sequel to To Kill a Mockingbird – and the feelings of the author of the American classic about plans to publish it.

Tonja Carter said Lee, 88, was “extremely hurt and humiliated” by reports she had been being duped by people who wanted to publish the work, Go Set a Watchman, without her approval.

Harper Lee book news leaves home town surprised, bemused and sceptical

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In a series of emails and texts, Carter told the New York Times Lee was “a very strong, independent and wise woman who should be enjoying the discovery of her long lost novel. Instead she is having to defend her own credibility and decision-making.”

Carter said she found the manuscript of Go Set a Watchman late last summer and at first assumed it was a copy of To Kill a Mockingbird.

“I was so stunned. At the time I didn’t know if it was finished,” Carter told the Times, recalling that she went to see Lee to ask if the novel was complete.

“Complete? I guess so. It was the parent of Mockingbird,” Carter said Lee told her. Lee reportedly thought the manuscript had long been lost.

News of the discovery broke last week. The publisher HarperCollins said it planned to publish Go Set a Watchman later this year. Lee has not published another book after the staggering success of To Kill a Mockingbird, which was published 55 years ago.

Joy, however, was accompanied by scepticism in some quarters, with questions about whether Lee understood and approved of what was happening.

Friends and neighbours in the tiny town of Monroeville, Alabama, which is the basis for Mockingbird’s Maycomb and where Lee lives in an assisted-living facility, spoke of the reclusive author being adamant that she never wanted to publish anything else and said she may not be able to stop others from doing so.

Even a statement from Lee that she was “happy as hell with the reactions to Watchman” did not quell doubts. There was also talk that the author may have been left vulnerable to unscrupulous business associates since the death of her sister Alice in November, at the age of 103.

The New York Times also quoted Cynthia McMillan, a resident assistant who has taken care of Lee for several years and who said the author was “sharp as a tack”. McMillan said: “She seems excited about [the publication of Go Set a Watchman], and it has given her something to focus on since her sister died.”

Book publisher Harper plans an initial run of two million copies of Go Set a Watchman, which follows one of the lead characters of To Kill a Mockingbird, the feisty Scout, as an adult.